1. Field of the Invention
This invention relates to remotely controlled apparatus insertable in the narrow gap between the rotor and stator in an electric power generator to measure the tightness of stator wedges held in place by ripple springs. More particularly, it relates to such apparatus which includes a non-conductive, non-magnetic probe mounted on a low profile carriage and extendable through holes in selected stator wedges to measure the deflection of the ripple springs.
2. Background Information
The stator coils in a large electric generator are retained in slots in the stator by stator wedges. Ripple springs inserted between the wedges and the coils maintain a positive load on the coils for slot tightness. It is very important that stator wedge tightness be maintained. Failure to maintain proper tightness can lead to stator coil insulation failure due to vibration caused by mechanical and magnetic loading.
In accordance with an accepted industry technique for checking for stator wedge tightness, a technician strikes the wedge, feels the vibration in the fingers, and listens to the sound. Through experience, technicians have learned to detect the characteristic feel and sound of a loose wedge.
In some generators, the stator wedges have been provided with a series of holes through which a depth micrometer is inserted. The holes are spaced along the wedge so that the difference between the readings on the depth micrometer can be used to calculate the peak to peak deflection of a ripple spring. A low differential indicates a tight wedge, while a differential greater than a selected amount indicates an unacceptably loose stator wedge.
While quite accurate, manually measuring the depth of the ripple spring at each hole is very time consuming. For instance, for a generator having 405 stator slots with three spaced wedges along each slot provided with 7 holes each, 8505 ripple spring readings must be made and recorded.
Both the traditional method in which a technician taps the stator wedges and feels as well as listens to the response to make a judgment on stator wedge tightness, and the above method in which a micrometer is used to measure ripple spring deflection, require removal of the rotor. This in itself can take two or three days. While it would be desirable to be able to use remotely controlled apparatus to check stator wedge tightness without removing the rotor, there is as little as only one and one-half inches between the stator and the rotor retainer ring with the rotor in place through which the test apparatus can be inserted.
U.S. patent application Ser. No. 07/277,472 filed on Nov. 23, 1988 and now Patent No. 4,889,000 discloses apparatus for testing stator wedge tightness which utilizes a low profile remotely controlled carriage on which is mounted a solenoid operated impactor. An on-board television camera is used to position the carriage with the impactor aligned with a wedge. The impactor strikes the wedge and a microphone mounted on the carriage records the resulting vibrations. It has been found that it is difficult to analyze the acoustic response with a computer for automatic testing for wedge tightness.
U.S. Pat. No. 4,803,563 discloses a remotely controlled carriage which is guided along the gap between the rotor and stator of an electric generator while an eddy current tester inspects the condition of the stator insulation. The carriage is held in place against the stator by permanent magnets embedded in the fiberglass chassis of the carriage.
Despite these improvements in inspection apparatus and techniques, there remains a need for improved apparatus for inspecting for generator stator wedge tightness.
There is a particular need for apparatus which can perform such an inspection with the accuracy of the depth micrometer test, but without the need for removal of the rotor.
There is a further need for such apparatus which can accommodate for sizable tolerances in the location of the holes in the stator wedges through which depth measurements must be made.
There is also a need for such apparatus which can compensate for the wide variation in the depth to be measured due to the undulations in the ripple springs.